Polyimide film

ABSTRACT

The present invention provides a polyimide film which is both outstandingly transparent and highly heat resistance, and which can be usefully employed as a transparent electrically conductive film, a TFT substrate, a flexible printed circuit substrate, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. Non-Provisionalapplication Ser. No. 13/142,761 (allowed), filed on Sep. 30, 2011, whichis a National Stage Entry of PCT/KR2009/007946, filed on Dec. 30, 2009;Korean Application No. 10-2008-0136314, filed on Dec. 30, 2008 andKorean Application no. 10-2009-0132417, filed Dec. 29, 2009, from whichpriority has been claimed in the prior application, is considered partof the disclosure of the accompanying Divisional application and ishereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a polyimide film, which is colorlessand transparent, and has an excellent heat resistant.

BACKGROUND ART

A polyimide resin, which is an insoluble and nonmeltable resin havingultrahigh heat resistance, is widely used in heat-resistantstate-of-the-art materials for automobiles, aircrafts, spacecrafts, andthe like, and electronic materials, such as insulating coating agents,insulating films, semiconductors, electrode protection films of TFT-LCD,and the like, because it has excellent properties, such as heatoxidation resistance, heat resistance, radiation resistance,low-temperature characteristics, chemical resistance, and the like.Recently, it is used in a transparent electrode film, and the like bysurface-coating or containing a conductive filler in a film and markingmaterial, such as optical fiber or liquid crystal alignment layer.

However, a general polyimide resin is colored brown or yellow because ofits high aromatic ring density so that it has low transmittance in thevisible light range and comes in some colors like yellow therebyreducing light transmittance with the result that it is difficult to useit in applications requiring transparency.

For this reason, there are diversely efforts to improve a color and atransmittance of a polyimide film, but there are two aspects as theresults of the above efforts, that is, a heat resistance is reduced inproportion to the improvements of color and transmittance of film.

Furthermore, a transparent film having high heat resistance as well as adiversification of function is required for applications as variouselectrical and electronic materials applied with a polyimide film.

DISCLOSURE Technical Problem

An object of the present invention is to provide a polyimide film havingan excellent heat resistance while it is contented with a transparency.

An embodiment of the present invention provides a polyimide powder,which is imides of polyamic acid obtained by polymerizing diamines andacid dianhydrides, in which it has an absolute molecular weight (Mw) of40,000 to 150,000, determined by the following Formula 1, and a degreeof imidization of at least 80%:

$\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

[wherein,

the above Formula 1 is induced from a principle determining molar massand size of polymer from angular variation and amount of scattered lightmeasured by an irradiation of laser light to a solution containing anypolymer and solvent, using a principle that the amount of chargetransfer and the amount of light emission are depended on apolarizability of material for the phenomenon, of which a polarizing isoccurred according to an interaction between the material and light, andhence light is scattered in all directions by oscillating charges;

R_(θ) is the excess Rayleigh ratio;

K*=4π²n₀ ²(dn/dc)²λ₀ ⁻⁴N_(A) ⁻¹, here n₀ is a refractive index ofsolvent, N_(A) is Avogadro's number, and dn/dc is a specific refractiveindex increment, that is differentiated from a change rate of refractiveindex changed according to a change rate of concentration of dilutesolution and is determined within the range of 0.001 to 0.1 g/ml as aconcentration change section, when a refractive index is detected byinjecting a polyimide powder in a state of dilute solution in an organicsolvent into a flow cell of differential refractometer;

c is a polymer concentration (g/ml) in a solution;

M is molar mass, and weight average molecular weight (Mw) in the case ofpolydisperse sample;

A₂ is the second virial coefficient; and

P(θ)=R_(θ)/R₀]

The polyimide powder according to an embodiment of the present inventionmay have an absolute molecular weight (Mw) of 50,000 to 150,000.

For the polyimide powder according to an embodiment of the presentinvention, its polydispersity index of absolute molecular weight may be1.1 to 1.5, and for a preferable embodiment, it may be 1.1 to 1.3.

For the polyimide powder according to an embodiment of the presentinvention, acid dianhydrides may include2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. At this time,2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride may beincluded in 30 mol % to 100 mol % in the acid dianhydrides.

For the polyimide powder according to an embodiment of the presentinvention, diamines may include2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl. At this time,2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl may be included in 20 mol% to 100 mol % in the diamines.

For the polyimide powder according to an embodiment of the presentinvention, the imides of polyamic acid may be imides of polyamic acidobtained by firstly injecting2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride out of theacid dianhydrides.

For the polyimide powder according to another embodiment of the presentinvention, the imides of polyamic acid may be imides of polyamic acidobtained by finally injecting2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride out of theacid dianhydrides.

For the polyimide powder according to an embodiment of the presentinvention, a polymerization may be performed for 1 hour to 24 hours.

For the polyimide powder according to an embodiment of the presentinvention, the polymerization may be performed for 8 hours to 12 hours.

An embodiment of the present invention provides a method for producing apolyimide powder, comprising:

obtaining a polyamic acid solution by polymerizing diamines and aciddianhydrides in an organic solvent;

producing a solution containing imides by an imidization to be thedegree of imidization of at least 80% by injecting a chemical conversionagent to the polyamic acid solution;

precipitating by adding a solvent selected from methyl alcohol, ethylalcohol, isopropyl alcohol, ethylene glycol, triethylene glycol, 2-butylalcohol, 2-propyl alcohol, 2-hexyl alcohol, cyclopentyl alcohol,cyclohexyl alcohol, phenol, and t-butyl alcohol to the solutioncontaining imides; and

filtering solids precipitated from the above step;

in which the polyimide powder has the degree of imidization of at least80% and an absolute molecular weight (Mw) of 40,000 to 150,000,determined by the following Formula 1:

$\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

[wherein,

the above Formula 1 is induced from a principle determining molar massand size of polymer from angular variation and amount of scattered lightmeasured by an irradiation of laser light to a solution containing anypolymer and solvent, using a principle that the amount of chargetransfer and the amount of light emission are depended on apolarizability of material for the phenomenon, of which a polarizing isoccurred according to an interaction between the material and light, andhence light is scattered in all directions by oscillating charges;

R_(θ) is the excess Rayleigh ratio;

K*=4π²n₀ ²(dn/dc)²λ₀ ⁻⁴N_(A) ⁻¹, here n₀ is a refractive index ofsolvent, N_(A) is Avogadro's number, and dn/dc is a specific refractiveindex increment, that is differentiated from a change rate of refractiveindex changed according to a change rate of concentration of dilutesolution and is determined within the range of 0.001 to 0.1 g/ml as aconcentration change section, when a refractive index is detected byinjecting a polyimide powder in a state of dilute solution in an organicsolvent into a flow cell of differential refractometer;

c is a polymer concentration (g/ml) in a solution;

M is molar mass, and weight average molecular weight (Mw) in the case ofpolydisperse sample;

A₂ is the second virial coefficient; and

P(θ)=R_(θ)/R₀]

Another embodiment of the present invention provides the polyimide filmobtained by producing a film with imides of polyamic acid obtained fromthe polymerization of diamines and acid dianhydrides, in which thepolymide film has an absolute molecular weight (Mw) of 30,000 to170,000, determined by the following Formula 1:

$\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

[wherein,

the above Formula 1 is induced from a principle determining molar massand size of polymer from angular variation and amount of scattered lightmeasured by an irradiation of laser light to a solution containing anypolymer and solvent, using a principle that the amount of chargetransfer and the amount of light emission are depended on apolarizability of material for the phenomenon, of which a polarizing isoccurred according to an interaction between the material and light, andhence light is scattered in all directions by oscillating charges;

R_(θ) is the excess Rayleigh ratio;

K*=4π²n₀ ²(dn/dc)²λ₀ ⁻⁴N_(A) ⁻¹, here n₀ is a refractive index ofsolvent, N_(A) is Avogadro's number, and dn/dc is a specific refractiveindex increment, that is differentiated from a change rate of refractiveindex changed according to a change rate of concentration of dilutesolution and is determined within the range of 0.001 to 0.1 g/ml as aconcentration change section, when a refractive index is detected byinjecting a polyimide film in a state of dilute solution in an organicsolvent into a flow cell of differential refractometer;

c is a polymer concentration (g/ml) in a solution;

M is molar mass, and weight average molecular weight (Mw) in the case ofpolydisperse sample;

A₂ is the second virial coefficient; and

P(θ)=R_(θ)/R₀]

The polyimide film according to an embodiment of the present inventionmay have the degree of imidization of at least 95%.

The polyimide film according to an embodiment of the present inventionmay have an absolute molecular weight (Mw) of 50,000 to 150,000.

For the polyimide film according to an embodiment of the presentinvention, its polydispersity index of absolute molecular weight may be1.1 to 1.6, and preferably may be 1.1 to 1.3.

For the polyimide film according to an embodiment of the presentinvention, the acid dianhydrides may include2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. At this time,2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride may beincluded in 30 mol % to 100 mol % in the acid anhydrides.

For the polyimide film according to an embodiment of the presentinvention, the diamines may include2,2′-bis(trifluoromethyl)-4,4′-diaminophenyl. At this time,2,2′-bis(trifluoromethyl)-4,4′-diaminophenyl may be included in 20 mol %to 100 mol % in the diamines.

The polyimide film according to an embodiment of the present inventionmay be obtained from imides of polyamic acid obtained by firstlyinjecting 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride outof the acid anhydrides.

The polyimide film according to another embodiment of the presentinvention may be obtained from imides of polyamic acid obtained byfinally injecting 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride out of the acid anhydrides.

For the polyimide film according to embodiments of the presentinvention, a polymerization may be performed for 1 hour to 24 hours.

Preferably, the polymerization may be performed for 8 hours to 12 hours.

The polyimide film according to an embodiment of the present inventionmay have a degree of yellowness of not more than 4.5 based on a filmthickness of 50˜100 μm.

Furthermore, the polyimide film according to an embodiment of thepresent invention may have a mean coefficient of linear thermalexpansion (CTE) of not more than 70 ppm/° C., measured within the rangeof 50 to 250° C. using a mechanical thermal analysis based on a filmthickness of 50˜100 μm.

Furthermore, an embodiment of the present invention provides a methodfor producing the polyimide film, comprising:

obtaining a polyamic acid solution by polymerizing diamines and aciddianhydrides in an organic solvent;

producing a solution containing imides by an imidization to be thedegree of imidization of at least 80% by injecting a chemical conversionagent to the polyamic acid solution;

precipitating by adding a solvent selected from methyl alcohol, ethylalcohol, isopropyl alcohol, ethylene glycol, triethylene glycol, 2-butylalcohol, 2-propyl alcohol, 2-hexyl alcohol, cyclopentyl alcohol,cyclohexyl alcohol, phenol, and t-butyl alcohol to the solutioncontaining imides;

filtering solids precipitated from the above step;

obtaining a polyimide powder by drying a filtrate;

dissolving the polyimide powder in an organic solvent;

making a film with the polyimide solution; and

heating the film at 100 to 500° C.;

in which the polyimide film has an absolute molecular weight (Mw) of30,000 to 170,000, determined by the following Formula 1:

$\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

[wherein,

the above Formula 1 is induced from a principle determining molar massand size of polymer from angular variation and amount of scattered lightmeasured by an irradiation of laser light to a solution containing anypolymer and solvent, using a principle that the amount of chargetransfer and the amount of light emission are depended on apolarizability of material for the phenomenon, of which a polarizing isoccurred according to an interaction between the material and light, andhence light is scattered in all directions by oscillating charges;

R_(θ) is the excess Rayleigh ratio;

K*=4π²n₀ ²(dn/dc)²λ₀ ⁻⁴N_(A) ⁻¹, here n₀ is a refractive index ofsolvent, N_(A) is Avogadro's number, and dn/dc is a specific refractiveindex increment, that is differentiated from a change rate of refractiveindex changed according to a change rate of concentration of dilutesolution and is determined within the range of 0.001 to 0.1 g/ml as aconcentration change section, when a refractive index is detected byinjecting a polyimide film in a state of dilute solution in an organicsolvent into a flow cell of differential refractometer;

c is a polymer concentration (g/ml) in a solution;

M is molar mass, and weight average molecular weight (Mw) in the case ofpolydisperse sample;

A₂ is the second virial coefficient; and

P(θ)=R_(θ)/R₀]

For the method for producing the polyimide film according to anembodiment of the present invention, the chemical conversion agent mayinclude a catalyst and a dehydrating agent.

The polyimide film according to an embodiment of the present inventionhas the excellent transparency and heat resistance, and hence thedimensions change according to thermal stress is small, so that it isexpected to be useful in a transparent electrically conductive film, aTFT substrate, a flexible printed circuit substrate, and the like.

Technical Solution

Hereinafter, the present invention will be described in more detail asfollows.

The polyimide powder according to an embodiment of the present inventionmay be imides of polyamic acid obtained by the polymerization ofdiamines and acid dianhydrides, in which the degree of imidization maybe at least 80% and its absolute molecular weight (Mw) may be 40,000 to150,000 in order to secure a transparency and also satisfy a heatresistance, in which the absolute molecular weight is determined by thefollowing Formula 1:

$\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

[wherein,

the above Formula 1 is induced from a principle determining molar massand size of polymer from angular variation and amount of scattered lightmeasured by an irradiation of laser light to a solution containing anypolymer and solvent, using a principle that the amount of chargetransfer and the amount of light emission are depended on apolarizability of material for the phenomenon, of which a polarizing isoccurred according to an interaction between the material and light, andhence light is scattered in all directions by oscillating charges;

R_(θ) is the excess Rayleigh ratio;

K*=4π²n₀ ²(dn/dc)²λ₀ ⁻⁴N_(A) ⁻¹, here n₀ is a refractive index ofsolvent, N_(A) is Avogadro's number, and dn/dc is a specific refractiveindex increment, that is differentiated from a change rate of refractiveindex changed according to a change rate of concentration of dilutesolution and is determined within the range of 0.001 to 0.1 g/ml as aconcentration change section, when a refractive index is detected byinjecting a polyimide powder in a state of dilute solution in an organicsolvent into a flow cell of differential refractometer;

c is a polymer concentration (g/ml) in a solution;

M is molar mass, and weight average molecular weight (Mw) in the case ofpolydisperse sample;

A₂ is the second virial coefficient; and

P(θ)=R_(θ)/R₀]

For instance, a method for measuring an absolute molecular weight formeasuring a molecular weight of polymer may include a method formeasuring an absolute molecular weight using light scattering in apolymer solution.

The light scattering is occurred by a polymer chain in the polymersolution because a polymer coil size is smaller or similar to wavelengthof light, or also the polymer chains is polarized by an electric fieldof incident light. A degree of scattering is out of proportion to theamount of material that causes scattering, and in the case of the sameamount of scatterer, the scattering caused by large particles is verystrong rather than that caused by small particles. Accordingly, thedegree of light scattering is influenced by the particle size so thatthe information about the molecular weight of polymer can be obtained byusing the degree of light scattering. Furthermore, when light passthrough the dilute polymer solution, of which a refractive index ofsolvent is different from a refractive index of polymer dissolved in theabove solvent, light may be scattered according to strength depended onthe size and concentration of polymer dissolved in addition to thedifference of refractive indexes of the polymer and solvent. If thepolymer solution is sufficient dilute solution, the strength ofscattered light may be expressed as the total levels of contributionsfor scattering occurred by each of the polymer coils that are wellseparated in the solution. That is because when the size of polymer coildissolved is significantly small rather than wavelength of light, theymay be isotropy, or when having the same polarizability in alldirections, the strength of scattered light by each of the polymer coilsin any direction is proportional to the square of wave vector size ofscattered light.

The above Formula 1 is induced from the above principle, and examples ofapparatus for obtaining an absolute molecular weight by the formula asmentioned above include Multi Angle Light Scattering (MALS) Systemproduced from Wyatt Company. Many data in addition to a weight averagemolecular weight, a size, a molecular weight distribution, and the likeof samples to be analyzed can be obtained from MALS as mentioned above.

However, it is generally difficult to measure an absolute molecularweight according to light scattering in the case of the polyimide powderor polyimide film because a solubilization of polymer is difficult dueto a large quantity of aromatic rings. When a large quantity of aromaticrings is presented, it becomes to be colored.

In this way, in the case of the polyimide powder having an absolutemolecular weight (Mw) of 40,000 to 150,000 obtained by MALS suppliedfrom an embodiment of the present invention, the transparency and alsoheat resistance are excellent.

If the absolute molecular weight (Mw) obtained by MALS is not more than40,000, a filming may not be occurred due to a lack of viscosity, or anoptical property, a mechanical property, and heat resistance may bereduced; and if it is more than 150,000, a thickness of film isdifficult to control due to an excessive viscosity, or physicalproperties may be different according to film or part of film, and aflexibility and a productivity may be reduced on producing a film.Preferably, the absolute molecular weight of 50,000 to 150,000 may bepreferable from the transparency and heat resistance viewpoints.

Furthermore, when the degree of imidization of the polyimide powder ofthe present invention is at least 80%, it is preferable from a storagestability viewpoint. When the degree of imidization of the polyimidepowder is not more than 80%, there may be a problem with the storagestability.

Furthermore, the polydispersity index of absolute molecular weight ofthe polyimide powder according to an embodiment of the presentinvention, in which the polydispersity index of absolute molecularweight is determined by Formula 1, may be 1.1 to 1.5. Preferably, thepolydispersity index of absolute molecular weight may be within theabove range because it affects the optical property, the mechanicalproperty, and the heat resistance. Most preferably, the polydispersityindex of absolute molecular weight may be 1.1 to 1.3.

Example for obtaining the polyimide powder that can be contented withthe degree of imidization and absolute molecular weight as mentionedabove may include a method for controlling a selection of monomer, anorder of polymerization, a method for polymerizing, and the like, andalso a method for changing a precipitating method for obtaining thepowder. For example, the polyimide powder according to an embodiment ofthe present invention may be obtained by the imidization of the polyamicacid obtained from the polymerization of the acid dianhydrides anddiamines.

Considering the transparency, preferably the acid dianhydrides mayinclude 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride(6-FDA). And, it may further include more than one selected from thegroup consisting of4-(2,5-dioxotetrahydrofurane-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylicanhydride (TDA), and4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalicanhydride) (HBDA).Considering the heat resistance, more preferably more than one selectedfrom pyromellitic dianhydride (PMDA), biphenyltetracarboxylicdianhydride (BPDA), and oxyphthalic dianhydride (ODPA) may be usedtogether.

The used amount of 6-FDA out of the acid dianhydrides may be preferably30 mol % to 100 mol %.

Preferably, the amount of 6-FDA used of the acid dianhydrides may be 30to 100 mol % for expressing the transparency and hindering otherphysical properties, such as the heat resistance.

Meanwhile, example of the diamines may include more than one selectedfrom 2,2-bis[4-(4-aminophenoxy)-phenyl]propane (6HMDA),2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (2,2′-TFDB),3,3′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (3,3′-TFDB),4,4′-bis(3-aminophenoxy)diphenylsulfone (DBSDA),bis(3-aminophenyl)sulfone (3DDS), bis(4-aminophenyl)sulfone (4DDS),1,3-bis(3-aminophenoxy)benzene (APB-133), 1,4-bis(4-aminophenoxy)benzene(APB-134), 2,2′-bis[3(3-aminophenoxy)phenyl]hexafluoropropane (3-BDAF),2,2′-bis[4(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF),2,2′-bis(3-aminophenyl)hexafluoropropane (3,3′-6F),2,2′-bis(4-aminophenyl)hexafluoropropane (4,4′-6F), and oxydianiline(ODA), and preferably may include 2,2′-TFDB of the whole diamines forproperly securing a free volume by a side chain.

More preferably, 2,2′-TFDB of the whole diamines may be included in 20to 100 mol % for maintaining the transparency through securing the freevolume by the side chain.

The equimolar concentration of the acid dianhydride components anddiamine components as mentioned above are dissolved in a solvent toreact and then the polyamic acid solution is produced.

For the reaction conditions, it is not limited, but the reactiontemperature is preferably −20˜80° C., and the polymerization time is 1hour to 24 hours, but preferably 8 hours to 12 hours. Furthermore, morepreferably, the reaction is performed under an inert atmosphere, such asargon, nitrogen, and the like.

Example of the solvent for the solution polymerization of the monomersas mentioned above (hereinafter, referred to as “first solvent”) is notlimited especially as long as it can dissolve the polyamic acid. Morethan one polar solvent selected from m-cresol, N-methyl-2-pyrrolidone(NMP), dimethylformamide (DMF), dimethylacetamide (DMAc),dimethylsulfoxide (DMSO), acetone, and diethylacetate is used as theknown reaction solvent. A low boiling point solution, such astetrahydrofuran (THF) and chloroform, and a low absorbency solvent, suchas γ-butyrolactone can be used above this.

The content of the first solvent is not limited especially, butpreferably 50˜95 wt % in the whole polyamic acid solution, and morepreferably 70˜90 wt % in order to obtain the proper molecular weight andviscosity of the polyamic acid solution.

A method for producing the polyimide powder using the monomer asmentioned above is not limited especially, and for example the polyimideresin solids can be obtained by the method comprising: obtaining thepolyamic acid solution by polymerizing diamines and acid dianhydridesunder the first solvent; producing the solution containing imides by theimidization of the polyamic acid solution obtained; precipitating byadding the second solvent to the solution containing the imides; andfiltering and drying the solids precipitated.

At this time, a polarity of the second solvent may be lower than that ofthe first solvent, and the second solvent is for precipitating the resinsolids.

Example of the second solvent may include water, methyl alcohol, ethylalcohol, isopropyl alcohol, ethylene glycol, triethylene glycol, 2-butylalcohol, 2-propyl alcohol, 2-hexyl alcohol, cyclopentyl alcohol,cyclohexyl alcohol, phenol, t-butyl alcohol, and the like.

Meanwhile, the heat resistance of the polyimide can be ultimatelycontrolled by controlling the injection order of monomers, and forexample preferably, the molecular weight may be increased and also thepolyimide powder having greater absolute molecular weight may beobtained under the same time period of the polymerization by injecting6-FDA out of acid dianhydrides at the end, not by injecting it inadvance. As a result, the heat resistance can be controlled bycontrolling the injection order of monomers so that the heat resistancecan be improved in the case of the polyimide powder having high absolutemolecular weight.

In addition, the heat resistance of the film can be controlled accordingto the polymerization time, that is, the longer the polymerization time,the larger the absolute molecular weight. However, when a fixedpolymerization time has passed, the value of the absolute molecularweight would again get smaller so that it can be expected that thepolymerization time becomes excessively longer, and hence the absolutemolecular weight is decreased due to a depolymerization.

Therefore, if the polymerization time becomes excessively longer, thethermal stability (CTE) may be deteriorated due to the decrease of themolecular weight, while if it becomes excessively shorter; thedistribution of the molecular weight (PDI) becomes excessively broaderthereby reducing the mechanical physical properties of the film.Accordingly, preferably, the polymerization time may be 1 hour to 24hours, and most preferably 8 hours to 12 hours in order to giving theproper absolute molecular weight value and distribution of the absolutemolecular weight so that the polyimide powder can be obtained to evenlysatisfy the heat resistance and transparency.

When imidizing by injecting the chemical conversion agent to thepolyamic acid solution, the degree of imidization may be at least 80%,and preferably at least 85% for the optical properties, mechanicalproperties, and heat resistance.

For the conditions of drying the polyimide resin solids obtained afterfiltering, preferably the temperature may be 50˜120° C. and the time maybe 3 hours to 24 hours considering the boiling point of the secondsolvent.

Meanwhile, according to an embodiment of the present invention, theimides of the polyamic acid obtained by polymerizing diamines and aciddianhydrides may be obtained by making a film, and the polyimide filmhaving the absolute molecular weight of 30,000 to 170,000 can beprovided, in which the absolute molecular weight is determined by thefollowing Formula 1:

$\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

[wherein,

the above Formula 1 is induced from a principle determining molar massand size of polymer from angular variation and amount of scattered lightmeasured by an irradiation of laser light to a solution containing anypolymer and solvent, using a principle that the amount of chargetransfer and the amount of light emission are depended on apolarizability of material for the phenomenon, of which a polarizing isoccurred according to an interaction between the material and light, andhence light is scattered in all directions by oscillating charges;

R_(θ) is the excess Rayleigh ratio;

K*=4π²n₀ ²(dn/dc)²λ₀ ⁻⁴N_(A) ⁻¹, here n₀ is a refractive index ofsolvent, N_(A) is Avogadro's number, and dn/dc is a specific refractiveindex increment, that is differentiated from a change rate of refractiveindex changed according to a change rate of concentration of dilutesolution and is determined within the range of 0.001 to 0.1 g/ml as aconcentration change section, when a refractive index is detected byinjecting a polyimide film in a state of dilute solution in an organicsolvent into a flow cell of differential refractometer;

c is a polymer concentration (g/ml) in a solution;

M is molar mass, and weight average molecular weight (Mw) in the case ofpolydisperse sample;

A₂ is the second virial coefficient; and

P(θ)=R_(θ)/R₀]

As mentioned above, it is generally difficult to measure the absolutemolecular weight according to light scattering in the case of thepolyimide film because a solubilization of polymer is difficult due to alarge quantity of aromatic rings. When a large quantity of aromaticrings is presented, it becomes to be colored.

In this way, in the case of the polyimide film having the absolutemolecular weight (Mw) of 30,000 to 170,000 obtained by MALS suppliedfrom an embodiment of the present invention, the transparency and alsoheat resistance are excellent.

If the absolute molecular weight (Mw) of the polyimide film obtained byMALS, in which Mw is obtained by MALS, is not more than 30,000, theoptical property, the mechanical property, and the heat resistance maybe reduced; and if it is more than 170,000, the flexibility and theproductivity of the film may be reduced. Preferably, the absolutemolecular weight of 30,000 to 170,000 may be preferable from thetransparency and heat resistance viewpoints.

Furthermore, the degree of imidization of the polyimide film of thepresent invention is preferably at least 95% from the optical property,mechanical property, and the heat resistance viewpoints.

When the degree of imidization of the polyimide film is not more than95%, there may be a problem with the optical and mechanical physicalproperty, and the heat resistance.

Furthermore, the polydispersity index of absolute molecular weight ofthe polyimide film according to an embodiment of the present invention,in which the polydispersity index is determined by Formula 1, may be 1.1to 1.6. Preferably, the polydispersity index of absolute molecularweight may be within the above range because it affects the opticalphysical property, the mechanical physical property, and the heatresistance. Most preferably, the polydispersity index of absolutemolecular weight may be 1.1 to 1.3.

Example for obtaining the polyimide film that can be contented with thedegree of imidization and absolute molecular weight as mentioned abovemay include a method for controlling a selection of monomer, an order ofpolymerization, a method for polymerizing, and the like, or also amethod for selecting a precipitating method for obtaining the powder, asmentioned in the paragraphs of the polyimide powder. The detaileddescription related to the above methods will be omitted in here.

The method for producing the polyimide film may include steps fordissolving the polyimide powder obtained as mentioned above in anorganic solvent to obtain the polyimide solution, and then making andheating the film.

At this time, the first solvent can be used as the organic solvent.

The polyimide solution is casted on a support and then is heated for 1minute to 8 hours while it is slowly increased within the range of40˜400° C. to obtain the polyimide film. It may be further heated onceagain in order to increase the thermal stability and reduce a thermalhistory. The temperature for further heating may preferably be 100˜500°C. and the time for heating may preferably 1 minute to 30 minutes.

A residual volatile component in the film after heating may be not morethan 5%, and preferably not more than 3%.

At this time, the chemical conversion agent may include the dehydratingagent represented as acid anhydride, such as acetic acid anhydride, andthe like, and the catalyst for imidizing represented as tertiary amines,such as isoquinoline, β-picoline, pyridine, and the like. It may bepreferable to combine the chemical imidization for reducing the decreaseof the molecular weight.

Furthermore, preferably, the polyimide film according to an embodimentof the present invention may have the degree of yellowness of not morethan 4.5 for securing the transparency.

In addition, the average transmittance at 400 to 740 nm measured byusing UV spectrophotometer based on the film thickness of 50˜100 μm maybe preferably at least 85%. If the average transmittance at 400 to 740nm measured by using UV spectrophotometer based on the film thickness of50˜100 μm is not more than 85%, there may be a problem that a propervisual effect could not be shown for using as the application ofdisplay.

Furthermore, unlike the general colored polyimide film, the polyimidefilm according to an embodiment of the present invention may preferablyhave L value of at least 90, a value of not more than 5, and b value ofnot more than 5 when measuring a chromaticity coordinates by using UVspectrophotometer based on the film thickness of 50˜100 μm.

In addition, considering the effect to the dimension change, thepolyimide film may preferably have the mean coefficient of linearthermal expansion (CTE) of not more than 70 ppm/° C., measured withinthe range of 50 to 250° C. using the mechanical thermal analysis basedon a film thickness of 50˜100 μm. When the mean coefficient of linearthermal expansion is more than the above value, this causes thedimension change because the mean coefficient of linear thermalexpansion becomes excessively larger on producing an adhesive film andthe difference of coefficient of linear thermal expansion with that ofmetal film becomes great.

Preferably, the mean coefficient of linear thermal expansion (CTE) maybe 15 ppm/° C. to 60 ppm/° C.

BEST MODE

Hereinafter, the embodiments of the present invention will be describedin detail as follows, but the present invention will not be limitedthereto.

Example 1

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 71.08 g (0.16 mol) of 6FDA was added and stirred for1 hour to completely dissolve 6FDA. At this time, the temperature of thesolution was maintained at 25° C. And, 11.76 g (0.04 mol) of BPDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for12 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 120 g of the solid powder (the degreeof imidization was 82%).

Example 2

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 11.76 g (0.04 mol) of BPDA was added and stirred for1 hour to completely dissolve BPDA. At this time, the temperature of thesolution was maintained at 25° C. And, 71.08 g (0.16 mol) of 6FDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for3 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 90 g of the solid powder (the degreeof imidization was 80%).

Example 3

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 11.76 g (0.04 mol) of BPDA was added and stirred for1 hour to completely dissolve BPDA. At this time, the temperature of thesolution was maintained at 25° C. And, 71.08 g (0.16 mol) of 6FDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for12 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 126 g of the solid powder (the degreeof imidization was 82%).

Example 4

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 11.76 g (0.04 mol) of BPDA was added and stirred for1 hour to completely dissolve BPDA. At this time, the temperature of thesolution was maintained at 25° C. And, 71.08 g (0.16 mol) of 6FDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for24 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 125 g of the solid powder (the degreeof imidization was 83%).

The data about the polymer was collected using the following methodabout the polyimide powders that were obtained from the above Example 1to Example 4.

(1) Apparatus and Method for Analyzing

GPC & MALS Analysis Apparatus: GPC—Water 1525 Binary HPLC pump; RIdetector—Wyatt optilab rEX; MALS—Wyatt Dawn 8+; Column—μ—Styragel HTLinear (7.8*300 mm) 2EA, Styragel HT 6E

(2) Pretreatment Method of Sample

0.05 g of the powders that were obtained from the above Example 1 toExample 4 were weighted and dissolved in 10 ml of DMF (containing 0.05%LiCl). The solutions of DMF containing the powder were added to an ovenof 50° C. and dissolved for 2 hours while shaking. After completelydissolving the sample, it was filtered with 0.45 μm syringe filter andthen installed to MALS autosampler.

(3) Analysis Method

Injection volume: 400 μl

Injection Temp.: 50° C.

Flow Rate: 1 ml/min

Eluent: DMF (containing 0.05% LiCl): Refractive index 1.405

Column Temp.: 50° C.

Dn/Dc: see the following description

At this time, Dn/Dc relates to the specific refractive index increment,and is the value that a change rate of refractive index according to thechange rate of dilute solution concentration is differentiated and ismeasured within the range of 0.001 to 0.1 g/ml that is a section ofconcentration change when detecting a refractive index through injectingthe polyimide powder in a state of dilute solution in an organic solventinside flow cell of differential refractometer. Specifically, the abovevalue was measured as the following method.

(4) Analysis Apparatus that is Used for Measuring Dn/Dc

RI Detector: Wyatt Optilavb rEX

(5) Pretreatment Method of Sample for Measuring Dn/Dc

Firstly, 0.2 g of the polyimide powders that were obtained from theabove Example 1 to Example 4 were dissolved in 50 ml of DMF (containing0.05% LiCl) to prepare a sample of high concentration. At this time,because it was not easily dissolved, it was added to an oven of 50° C.,and dissolved for about 2 hours while shaking. The samples having 0.0032g/ml, 0.0024 g/ml, 0.0016 g/ml and 0.0008 g/ml concentration,respectively were prepared by diluting the sample having a highconcentration. For each sample, the refractive index values according tothe concentration were measured using 0.45 μm syringe filter.

(6) Analysis Method of Dn/Dc Sample

injection volume: 10 ml

injector Temp.: 50° C.

flow rate: 16 ml/hr

eluent: DMF (containing 0.05% LiCl)

As the results obtained from the above analysis, in the case of thepolyimide powders that were obtained from the above Example 1 to Example4, Dn/Dc value was 0.1180 at 50° C. of DMF (containing 0.05% LiCl).

The absolute molecular weight value according to MALS can be calculatedaccording to the above method from Dn/Dc value that was obtained. Theresults were shown in the following Table 1.

TABLE 1 Mn Mp Mw Mz Rz Dn/Dc (g/mol) (g/mol) (g/mol) (g/mol) (nm)Polydispesity Example 1 0.1180 5.004 × 10⁴ 7.513 × 10⁴ 5.655 × 10⁴ 6.132× 10⁴ 9.9 1.130 Example 2 0.1180 3.407 × 10⁴ 4.446 × 10⁴ 4.241 × 10⁴5.511 × 10⁴ 14.6 1.245 Example 3 0.1180 1.138 × 10⁵ 1.438 × 10⁵ 1.385 ×10⁵ 1.810 × 10⁵ 24.0 1.217 Example 4 0.1180 7.564 × 10⁴ 1.120 × 10⁵8.727 × 10⁴ 9.496 × 10⁴ 14.8 1.153

Example 5

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 71.08 g (0.16 mol) of 6FDA was added and stirred for1 hour to completely dissolve 6FDA. At this time, the temperature of thesolution was maintained at 25° C. And, 11.76 g (0.04 mol) of BPDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for3 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 120 g of the solid powder (the degreeof imidization was 80%).

The solid powder obtained from the above method was dissolved in 480 gof N,N-dimethylacetamide (DMAc) to obtain 20 wt % of the solution(viscosity: 70 poise).

After finishing the reaction, the obtained solution was applied to astainless board and then cast in 700 μm; after drying for 1 hour with ahot-air of 150° C., the film was detached from the stainless board andthen fixed in a frame with a pin.

After the frame fixed with the film was added to the vacuum oven andslowly heated for 2 hours from 100° C. to 300° C., it was slowly cooledto remove from the frame to obtain the polyimide film. And then, it wasagain heated for 30 minutes at 300° C. to obtain the polyimide film asthe final heating (thickness: 100 μm, and the degree of imidization:95%).

Example 6 to Example 8

The polyimide film was obtained using the same method to the aboveExample 5, but the reaction time was changed to 5, 12 and 24 hours,respectively when preparing the solution of the polyamic acid.

Example 9

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 11.76 g (0.04 mol) of BPDA was added and stirred for1 hour to completely dissolve BPDA. At this time, the temperature of thesolution was maintained at 25° C. And, 71.08 g (0.16 mol) of 6FDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for3 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 90 g of the solid powder (the degreeof imidization was 80%).

The solid powder obtained from the above method was dissolved in 360 gof N,N-dimethylacetamide (DMAc) to obtain 20 wt % of the solution(viscosity: 70 poise).

After finishing the reaction, the obtained solution was applied to astainless board and then cast in 700 μm; after drying for 1 hour with ahot-air of 150° C., the film was detached from the stainless board andthen fixed in a frame with a pin.

After the frame fixed with the film was added to the vacuum oven andslowly heated for 2 hours from 100° C. to 300° C., it was slowly cooledto remove from the frame to obtain the polyimide film. And then, it wasagain heated for 30 minutes at 300° C. to obtain the polyimide film asthe final heating (thickness: 100 μm, and the degree of imidization:95%).

Example 10 and Example 11

The polyimide film was obtained using the same method to the aboveExample 9, but the reaction time was changed to 12 and 24 hours,respectively when preparing the solution of the polyamic acid.

Example 12

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 11.76 g (0.04 mol) of BPDA was added and stirred for1 hour to completely dissolve BPDA. At this time, the temperature of thesolution was maintained at 25° C. And, 71.08 g (0.16 mol) of 6FDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for12 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of water to precipitate; theprecipitated solid was filtered and grinded; and then dried at 80° C. invacuum for 6 hours to obtain 123 g of the solid powder (the degree ofimidization was 81%).

The solid powder obtained from the above method was dissolved in 492 gof N,N-dimethylacetamide (DMAc) to obtain 20 wt % of the solution(viscosity: 70 poise).

After finishing the reaction, the obtained solution was applied to astainless board and then cast in 700 μm; after drying for 1 hour with ahot-air of 150° C., the film was detached from the stainless board andthen fixed in a frame with a pin.

After the frame fixed with the film was added to the vacuum oven andslowly heated for 2 hours from 100° C. to 300° C., it was slowly cooledto remove from the frame to obtain the polyimide film. And then, it wasagain heated for 30 minutes at 300° C. to obtain the polyimide film asthe final heating (thickness: 100 μm, and the degree of imidization:95%).

The data about the polymer was collected using the following methodabout the polyimide films that were obtained from the above Example 5 to

Example 12 (1) Apparatus and Method for Analyzing

GPC & MALS Analysis Apparatus: GPC—Water 1525 Binary HPLC pump; RIdetector—Wyatt optilab rEX; MALS—Wyatt Dawn 8+; Column—μ—Styragel HTLinear (7.8*300 mm) 2EA, Styragel HT 6E

(2) Pretreatment Method of Sample

0.05 g of the films that were obtained from the above Example 5 toExample 12 was weighted and 10 ml of DMF (containing 0.05% LiCl) wasadded in the vial. The solutions of DMF containing the films were addedto an oven of 50° C. and dissolved for 2 hours while shaking. Aftercompletely dissolving the sample, it was filtered with 0.45 μm syringefilter and then installed to MALS autosampler.

(3) Analysis Method

Injection volume: 400 μl

Injection Temp.: 50° C.

Flow Rate: 1 ml/min

Eluent: DMF (containing 0.05% LiCl, Refractive index 1.405)

Column Temp.: 50° C.

Dn/Dc: see the following description

At this time, Dn/Dc relates to the specific refractive index increment,and is the value that a change rate of refractive index according to thechange rate of dilute solution concentration is differentiated and ismeasured within the range of 0.001 to 0.1 g/ml that is a section ofconcentration change when detecting a refractive index through injectingthe polyimide film in a state of dilute solution in an organic solventinside flow cell of differential refractometer. Specifically, the abovevalue was measured as the following method.

(4) Analysis Apparatus that is Used for Measuring Dn/Dc

RI Detector: Wyatt Optilavb rEX

(5) Pretreatment Method of Sample for Measuring Dn/Dc

Firstly, 0.2 g of the polyimide films that were obtained from the aboveExample 5 to Example 12 were dissolved in 50 ml of DMF (containing 0.05%LiCl) to prepare a sample. At this time, because it was not easilydissolved, it was added to an oven of 50° C., and dissolved for about 2hours while shaking. The samples having 0.0032 g/ml, 0.0024 g/ml, 0.0016g/ml and 0.0008 g/ml concentration, respectively were prepared bydiluting the sample having a high concentration. The refractive indexvalues according to the concentration were measured using 0.45 μmsyringe filter.

(6) Analysis Method of Dn/Dc Sample

injection volume: 10 ml

injector Temp.: 50° C.

flow rate: 16 ml/hr

eluent: DMF (Refractive index 1.405)

As the results obtained from the above analysis, in the case of thepolyimide films that were obtained from the above Example 5 to Example12, Dn/Dc value was 0.1216 at 50° C. of DMF.

The absolute molecular weight value according to MALS can be calculatedaccording to the above method from Dn/Dc value that was obtained. Theresults were shown in the following Table 2.

Above this, the transmittance, a color coordinate, a degree ofyellowness and the coefficient of linear thermal expansion were measuredusing the following methods, and then the results were shown in thefollowing Table 3.

(7) Transmittance and Color Coordinate

The visible ray transmittances of the prepared films were measured usingUV spectrophotometer (available from Varian Company, Cary100).

In addition, the color coordinates of the prepared films were measuredaccording to ASTM E 1347-06 standard using UV spectrophotometer(available from Varian Company, Cary100), and an illuminant was based onthe measurement value according to CIE D65.

(8) Degree of Yellowness

The degree of yellowness was measured in ASTM E313 standard.

(9) Coefficient of Linear Thermal Expansion (CTE)

The mean coefficient of linear thermal expansion was measured at 50-250°C. according to TMA-Method using TMA (available from TA Instrumentcompany, Q400).

TABLE 2 Mn Mp Mw Mz Rz Dn/Dc (g/mol) (g/mol) (g/mol) (g/mol) (nm)Polydispesity Example 5 0.1216 1.454 × 10⁴ 1.606 × 10⁴ 1.734 × 10⁴ 2.052× 10⁴ 1.6 1.192 Example 6 0.1216 2.610 × 10⁴ 3.096 × 10⁴ 3.128 × 10⁴4.039 × 10⁴ 17.2 1.198 Example 7 0.1216 4.995 × 10⁴ 6.967 × 10⁴ 6.190 ×10⁴ 7.972 × 10⁴ 17.3 1.213 Example 8 0.1216 3.711 × 10⁴ 4.871 × 10⁴4.642 × 10⁴ 4.642 × 10⁴ 21.9 1.251 Example 9 0.1216 3.730 × 10⁴ 4.510 ×10⁴ 4.552 × 10⁴ 5.890 × 10⁴ 20.1 1.220 Example 10 0.1216 1.071 × 10⁵1.334 × 10⁵ 1.278 × 10⁵ 1.625 × 10⁵ 23.6 1.193 Example 11 0.1216 7.743 ×10⁴ 1.004 × 10⁵ 9.234 × 10⁴ 1.396 × 10⁵ 20.1 1.193 Example 12 0.12163.868 × 10⁴ 7.050 × 10⁴ 5.877 × 10⁴ 8.011 × 10⁴ 3.3 1.520

TABLE 3 Transmittance (%) Thickness CTE* 400 nm~ 550 nm~ ColorCoordinate (μm) (ppm/° C.) Y** 740 nm 740 nm 550 nm 500 nm 4200 nm L a bEx. 5 100 — 5.12 85.3 87.5 88.6 87.9 77.1 96.11 −0.95 3.03 6 100 53.63.97 87.8 90.9 90.4 89.6 80.0 96.08 −0.87 2.98 7 100 48.8 2.94 87.9 90.590.0 89.3 82.1 95.92 −0.59 2.25 8 100 44.2 2.78 87.9 90.4 89.9 89.3 82.595.9 −0.58 2.13 9 100 52.2 4.39 87.7 90.8 90.3 89.3 79.5 96.0 −0.90 3.2310 100 47.9 2.96 88.0 90.7 90.3 89.5 82.1 96.0 −0.62 2.28 11 100 51.22.85 88.0 90.6 90.2 89.5 82.2 96.0 −0.61 2.2 12 100 54.3 3.55 87.7 90.390.1 89.5 80.6 96.0 −0.88 2.3

From the results of the above Table 3, the polyimide film according tothe present invention has an excellent transparency and also anexcellent dimensional stability about a thermal stress. However, itcould be known that the film having an excessively small absolutemolecular weight like Example 5 has a high degree of yellowness.

Example 13

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 605.6 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 2.9422 g (0.01 mol) of BPDA was added and stirred for1 hour to completely dissolve BPDA. At this time, the temperature of thesolution was maintained at 25° C. And, 84.41 g (0.19 mol) of 6FDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for12 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 147 g of the solid powder (the degreeof imidization was 80.5%).

The solid powder obtained from the above method was dissolved in 588 gof N,N-dimethylacetamide (DMAc) to obtain 20 wt % of the solution(viscosity: 70 poise).

After finishing the reaction, the obtained solution was applied to astainless board and then cast in 700 μm; after drying for 1 hour with ahot-air of 150° C., the film was detached from the stainless board andthen fixed in a frame with a pin.

After the frame fixed with the film was added to the vacuum oven andslowly heated for 2 hours from 100° C. to 300° C., it was slowly cooledto remove from the frame to obtain the polyimide film. And then, it wasagain heated for 30 minutes at 300° C. to obtain the polyimide film asthe final heating (thickness: 100 μm, and the degree of imidization:99.8%).

The data about the polymer was collected using the following methodabout the polyimide films.

(1) Apparatus and Method for Analyzing

GPC & MALS Analysis Apparatus: GPC—Water 1525 Binary HPLC pump; RIdetector—Wyatt optilab rEX; MALS—Wyatt Dawn 8+; Column—use by connectingwith Shodex K-803, K-804 and K-805

(2) Pretreatment Method of Sample

0.05 g of the films that were obtained were weighted and added in 10 mlvial of DMF (containing 0.05% LiCl). The solutions of DMF containing thefilms were added to an oven of 50° C. and dissolved for 2 hours whileshaking. After completely dissolving the sample, it was filtered with0.45 μm syringe filter and then installed to MALS autosampler.

(3) Analysis Method

Injection volume: 400 μl

Injection Temp.: 50° C.

Flow Rate: 1 ml/min

Eluent: DMF (containing 0.05% LiCl, Refractive index 1.390)

Column Temp.: 50° C.

Dn/Dc: see the following description

At this time, Dn/Dc relates to the specific refractive index increment,and is the value that a change rate of refractive index according to thechange rate of dilute solution concentration is differentiated and ismeasured within the range of 0.001 to 0.1 g/ml that is a section ofconcentration change when detecting a refractive index through injectingthe polyimide film in a state of dilute solution in an organic solventinside flow cell of differential refractometer. Specifically, the abovevalue was measured as the following method.

(4) Analysis Apparatus that is Used for Measuring Dn/Dc

RI Detector: Wyatt Optilavb rEX

(5) Pretreatment Method of Sample for Measuring Dn/Dc

Firstly, 0.2 g of the polyimide films that were obtained were dissolvedin 50 ml of DMF (containing 0.05% LiCl) to prepare a sample having ahigh concentration. At this time, because it was not easily dissolved,it was added to an oven of 50° C., and dissolved for about 2 hours whileshaking. The samples having 0.0032 g/ml, 0.0024 g/ml, 0.0016 g/ml and0.0008 g/ml concentration, respectively were prepared by diluting thesample having a high concentration. The refractive index valuesaccording to the concentration were measured using 0.45 μm syringefilter.

(6) Analysis Method of Dn/Dc Sample

injection volume: 10 ml

injector Temp.: 50° C.

flow rate: 16 ml/hr

eluent: DMF (containing 0.05% LiCl, Refractive index 1.390)

As the results obtained from the above analysis, in the case of thepolyimide films that were obtained, Dn/Dc value was 0.1348±0.0010 at 50°C. of DMF (containing 0.05% LiCl).

The absolute molecular weight value according to MALS can be calculatedaccording to the above method from Dn/Dc value that was obtained. Theresults were shown in the following Table 4.

Example 14 to Example 17

The film was prepared using the same method to the above Example 13, butthe mole % of BPDA to TFDB was changed when preparing the solution ofthe polyamic acid as the following Table 4.

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 13 according to the above method from Dn/Dc value that wasobtained. The results were shown in the following Table 4.

Example 18

The film was prepared using the same method to the above Example 13, butthe polyamic acid solution was only produced by performing a thermalcuring through using an azeotropic dehydrating agent, such as toluene,not performing a chemical curing through using pyridine and aceticanhydride.

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 13 according to the above method from Dn/Dc value that wasobtained. The results were shown in the following Table 4.

TABLE 4 TFDB to BPDA Mn Mp Mw Mz Rz mol % Dn/Dc (g/mol) (g/mol) (g/mol)(g/mol) (nm) Polydispesity Ex. 13 5 0.1348 ± 5.616 × 10⁵ 7.643 × 10⁴7.557 × 10⁴ 1.055 × 10⁶ 10.5 1.346 0.0010 Ex. 14 10 0.1158 ± 5.394 × 10⁴5.865 × 10⁴ 7.907 × 10⁴ 1.185 × 10⁵ 26.5 1.466 0.0006 Ex. 15 20 0.1246 ±8.740 × 10⁴ 1.020 × 10⁵ 1.085 × 10⁵ 1.472 × 10⁵ 20.1 1.241 0.0012 Ex. 1640 0.1284 ± 8.458 × 10⁴ 9.391 × 10⁴ 1.016 × 10⁵ 1.425 × 10⁵ 21.9 1.2020.0007 Ex. 17 50 0.1390 ± 8.769 × 10⁴ 9.258 × 10⁴ 1.037 × 10⁵ 1.433 ×10⁵ 21.3 1.183 0.0002 Ex. 18 5 0.1736 ± 9.814 × 10⁴ 1.232 × 10⁵ 1.255 ×10⁵ 1.738 × 10⁵ 20.1 1.278 0.0028

For the films obtained from the above Example 13 to 16, the degree ofyellowness was measured based on ASTM E313 standard and then the resultswere shown in the following Table 5.

TABLE 5 Degree of Yellowness Mean Transmittance Example 13 2.05 90.10Example 14 1.6522 90.08 Example 15 3.63 90.08 Example 16 3.07 90.06Example 17 3.40 89.50 Example 18 3.66 89.00

Example 19

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 71.08 g (0.16 mol) of 6FDA was added and stirred for1 hour to completely dissolve 6FDA. At this time, the temperature of thesolution was maintained at 25° C. And, 11.76 g (0.04 mol) of BPDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for3 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 120 g of the solid powder (the degreeof imidization was 81%).

The solid powder obtained from the above method was dissolved in 480 gof N,N-dimethylacetamide (DMAc) to obtain 20 wt % of the solution(viscosity: 70 poise).

After finishing the reaction, the obtained solution was applied to astainless board and then cast in 700 μm; after drying for 1 hour with ahot-air of 150° C., the film was detached from the stainless board andthen fixed in a frame with a pin.

After the frame fixed with the film was added to the vacuum oven andslowly heated for 2 hours from 100° C. to 300° C., it was slowly cooledto remove from the frame to obtain the polyimide film. And then, it wasagain heated for 30 minutes at 300° C. to obtain the polyimide film asthe final heating (thickness: 100 μm, and the degree of imidization:99%).

The data about the polymer was collected using the following methodabout the polyimide films.

(1) Apparatus and Method for Analyzing

GPC & MALS Analysis Apparatus: GPC—Water 1525 Binary HPLC pump; RIdetector—Wyatt optilab rEX; MALS—Wyatt Dawn 8+; Column—use by connectingwith Shodex K-803, K-804 and K-805

(2) Pretreatment Method of Sample

0.05 g of the films that were obtained were weighted and added in 10 mlvial of DMF (containing 0.05% LiCl). The solutions of DMF containing thefilms were added to an oven of 50° C. and dissolved for 2 hours whileshaking. After completely dissolving the sample, it was filtered with0.45 μm syringe filter and then installed to MALS autosample.

(3) Analysis Method

Injection volume: 400 μl

Injection Temp.: 50° C.

Flow Rate: 1 ml/min

Eluent: DMF (containing 0.05% LiCl, Refractive index 1.390)

Column Temp.: 50° C.

Dn/Dc: see the following description

At this time, Dn/Dc relates to the specific refractive index increment,and is the value that a change rate of refractive index according to thechange rate of dilute solution concentration is differentiated and ismeasured within the range of 0.001 to 0.1 g/ml that is a section ofconcentration change when detecting a refractive index through injectingthe polyimide film in a state of dilute solution in an organic solventinside flow cell of differential refractometer. Specifically, the abovevalue was measured as the following method.

(4) Analysis Apparatus that is Used for Measuring Dn/Dc

RI Detector: Wyatt Optilavb rEX

(5) Pretreatment Method of Sample for Measuring Dn/Dc

Firstly, 0.2 g of the polyimide films that were obtained were dissolvedin 50 ml of DMF (containing 0.05% LiCl) to prepare a sample having ahigh concentration. At this time, because it was not easily dissolved,it was added to an oven of 50° C., and dissolved for about 2 hours whileshaking. The samples having 0.0032 g/ml, 0.0024 g/ml, 0.0016 g/ml and0.0008 g/ml concentration, respectively were prepared by diluting thesample having a high concentration. For each sample, the refractiveindex values according to the concentration were measured using 0.45 μmsyringe filter.

(6) Analysis Method of Dn/Dc Sample

injection volume: 10 ml

injector Temp.: 50° C.

flow rate: 16 ml/hr

eluent: DMF (containing 0.05% LiCl, Refractive index 1.390)

As the results obtained from the above analysis, in the case of thepolyimide films that were obtained, Dn/Dc value was 0.1246±0.0012 at 50°C. of DMF (containing 0.05% LiCl).

The absolute molecular weight value according to MALS can be calculatedaccording to the above method from Dn/Dc value that was obtained. Theresults were shown in the following Table 6.

Example 20

The film was obtained by using the same method with the above Example19, but only the polyamic acid was produced and stirred for 5 hours, andthen pyridine and acetic anhydride were added thereto.

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 19. The results were shown in the following Table 6.

Example 21

The film was obtained by using the same method with the above Example19, but only the polyamic acid was produced and stirred for 12 hours,and then pyridine and acetic anhydride were added thereto.

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 19. The results were shown in the following Table 6.

Example 22

The film was obtained by using the same method with the above Example19, but only the polyamic acid was produced and stirred for 24 hours,and then pyridine and acetic anhydride were added thereto.

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 19. The results were shown in the following Table 6.

Example 23

While a nitrogen was passing through 1 L reactor comprising a stirrer, anitrogen injector, a dropping funnel, a thermostat and a cooler as areactor, 587.54 g of N,N-dimethylacetamide (DMAc) was filled into thereactor, and then the temperature of the reactor was adjusted at 25° C.And then, 64.046 g (0.2 mol) of TFDB was dissolved and maintained at 25°C. To the reactor, 11.76 g (0.04 mol) of BPDA was added and stirred for1 hour to completely dissolve BPDA. At this time, the temperature of thesolution was maintained at 25° C. And, 71.08 g (0.16 mol) of 6FDA wasadded and then the solution of the polyamic acid having the solidconcentration of 20 wt % was obtained.

The solution of the polyamic acid was stirred at a room temperature for1 hours; 31.64 g of pyridine and 40.91 g of acetic anhydride wereinjected and stirred for 30 minutes; and then after again stirring at80° C. for 1 hour, it is cooled at a room temperature; it was slowlyinjected to the container containing 20 L of methanol to precipitate;the precipitated solid was filtered and grinded; and then dried at 80°C. in vacuum for 6 hours to obtain 90 g of the solid powder (the degreeof imidization was 82%).

The solid powder obtained from the above method was dissolved in 360 gof N,N-dimethylacetamide (DMAc) to obtain 20 wt % of the solution(viscosity: 70 poise).

After finishing the reaction, the obtained solution was applied to astainless board and then cast in 700 μm; after drying for 1 hour with ahot-air of 150° C., the film was detached from the stainless board andthen fixed in a frame with a pin.

After the frame fixed with the film was added to the vacuum oven andslowly heated for 2 hours from 100° C. to 300° C., it was slowly cooledto remove from the frame to obtain the polyimide film. And then, it wasagain heated for 30 minutes at 300° C. to obtain the polyimide film asthe final heating (thickness: 100 μm, and the degree of imidization:95%).

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 19. The results were shown in the following Table 6.

Example 24

The film was obtained by using the same method with the above Example23, but only the polyamic acid was produced and stirred for 12 hours,and then pyridine and acetic anhydride were added thereto.

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 19. The results were shown in the following Table 6.

Example 25

The film was obtained by using the same method with the above Example23, but only the polyamic acid was produced and stirred for 24 hours,and then pyridine and acetic anhydride were added thereto.

The absolute molecular weight value according to MALS and Dn/Dc valueabout the obtained films can be calculated using the same method toExample 19. The results were shown in the following Table 6.

TABLE 6 Mn Mp Mw Mz Rz Dn/Dc (g/mol) (g/mol) (g/mol) (g/mol) (nm)Polydispersity Ex. 19 0.1246 ± 1.249 × 10⁴ 1.709 × 10⁴ 1.845 × 10⁴ 4.716× 10⁴ — 1.478 0.0012 Ex. 20 0.1246 ± 2.556 × 10⁴ 2.929 × 10⁴ 3.290 × 10⁴3.133 × 10⁵ 34.7 1.287 0.0012 Ex. 21 0.1246 ± 4.767 × 10⁴ 5.785 × 10⁴5.942 × 10⁴ 1.145 × 10⁵ 25 1.246 0.0012 Ex. 22 0.1246 ± 3.430 × 10⁴4.524 × 10⁴ 4.493 × 10⁴ 8.112 × 10⁴ 22.6 1.310 0.0012 Ex. 23 0.1246 ±1.803 × 10⁴ 2.363 × 10⁴ 2.395 × 10⁴ 3.161 × 10⁴ 12.9 1.328 0.0012 Ex. 240.1246 ± 9.427 × 10⁴ 1.098 × 10⁵ 1.162 × 10⁵ 1.575 × 10⁵ 25.2 1.2320.0012 Ex. 25 0.1246 ± 7.268 × 10⁴ 8.199 × 10⁴ 8.805 × 10⁴ 1.199 × 10⁵19.5 1.212 0.0012

From the above Table 6, it could be expected that the absolute molecularweights in the case of the films according to Example 19 and Example 23are excessively small so that their degrees of yellowness would beslightly high from the result of Table 3.

1. A polyimide powder that is imides of polyamic acid obtained bypolymerizing diamines and acid dianhydrides, wherein a degree ofimidization of the polyimide powder is at least 80%; an absolutemolecular weight (Mw) is 40,000 to 150,000; and the absolute molecularweight (Mw) is determined by the following Formula 1: $\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$ wherein, the above Formula 1 is induced from a principledetermining molar mass and size of polymer from angular variation andamount of scattered light measured by an irradiation of laser light to asolution containing any polymer and solvent, using a principle that theamount of charge transfer and the amount of light emission are dependedon a polarizability of material for the phenomenon, of which apolarizing is occurred according to an interaction between the materialand light, and hence light is scattered in all directions by oscillatingcharges; R_(θ) is the excess Rayleigh ratio; K*=4π²n₀ ²(dn/dc)²λ₀⁻⁴N_(A) ⁻¹, here n₀ is a refractive index of solvent, N_(A) isAvogadro's number, and dn/dc is a specific refractive index increment,that is differentiated from a change rate of refractive index changedaccording to a change rate of concentration of dilute solution and isdetermined within the range of 0.001 to 0.1 g/ml as a concentrationchange section, when a refractive index is detected by injecting apolyimide powder in a state of dilute solution in an organic solventinto a flow cell of differential refractometer; c is a polymerconcentration (g/ml) in a solution; M is molar mass, and weight averagemolecular weight (Mw) in the case of polydisperse sample; A₂ is thesecond virial coefficient; and P(θ)=R_(θ)/R₀.
 2. The polyimide powderaccording to claim 1, wherein the absolute molecular weight (Mw) is50,000 to 150,000.
 3. The polyimide powder according to claim 1, whereina polydispersity index of the absolute molecular weight is 1.1 to 1.5.4. The polyimide powder according to claim 3, wherein the polydispersityindex of the absolute molecular weight is 1.1 to 1.3.
 5. The polyimidepowder according to claim 1, wherein the acid dianhydrides include2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride.
 6. Thepolyimide powder according to claim 5, wherein2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride is included in30 mol % to 100 mol % in the acid dianhydrides.
 7. The polyimide powderaccording to claim 1, wherein the diamines include2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl.
 8. The polyimide powderaccording to claim 7, wherein2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl is included in 20 mol %to 100 mol % in the diamines.
 9. The polyimide powder according to claim5, wherein the imides of polyamic acid is obtained by firstly injecting2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride out of theacid dianhydride on polymerizing.
 10. The polyimide powder according toclaim 5, wherein the imides of polyamic acid is obtained by finallyinjecting 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride outof the acid dianhydride on polymerizing.
 11. The polyimide powderaccording to claim 1, wherein the polymerization is performed for 1 hourto 24 hours.
 12. The polyimide powder according to claim 11, wherein thepolymerization is performed for 8 hour to 12 hours.
 13. A method forproducing a polyimide powder, comprising: obtaining a polyamic acidsolution by polymerizing diamines and acid dianhydrides in an organicsolvent; producing a solution containing imides by an imidization to bethe degree of imidization of at least 80% by injecting a chemicalconversion agent to the polyamic acid solution; precipitating by addinga solvent selected from methyl alcohol, ethyl alcohol, isopropylalcohol, ethylene glycol, triethylene glycol, 2-butyl alcohol, 2-propylalcohol, 2-hexyl alcohol, cyclopentyl alcohol, cyclohexyl alcohol,phenol, and t-butyl alcohol to the solution containing imides; andfiltering solids precipitated from the above step; wherein the polyimidepowder has the degree of imidization of at least 80% and an absolutemolecular weight (Mw) of 40,000 to 150,000, determined by the followingFormula 1: $\begin{matrix}{\frac{R_{\theta}}{K^{*}c} = {{{MP}(\theta)} - {2A_{2}{cM}^{2}{P^{2}(\theta)}}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$ wherein, the above Formula 1 is induced from a principledetermining molar mass and size of polymer from angular variation andamount of scattered light measured by an irradiation of laser light to asolution containing any polymer and solvent, using a principle that theamount of charge transfer and the amount of light emission are dependedon a polarizability of material for the phenomenon, of which apolarizing is occurred according to an interaction between the materialand light, and hence light is scattered in all directions by oscillatingcharges; R_(θ) is the excess Rayleigh ratio; K*=4π²n₀ ²(dn/dc)²λ₀⁻⁴N_(A) ⁻¹, here n₀ is a refractive index of solvent, N_(A) isAvogadro's number, and dn/dc is a specific refractive index increment,that is differentiated from a change rate of refractive index changedaccording to a change rate of concentration of dilute solution and isdetermined within the range of 0.001 to 0.1 g/ml as a concentrationchange section, when a refractive index is detected by injecting apolyimide powder in a state of dilute solution in an organic solventinto a flow cell of differential refractometer; c is a polymerconcentration (g/ml) in a solution; M is molar mass, and weight averagemolecular weight (Mw) in the case of polydisperse sample; A₂ is thesecond virial coefficient; and P(θ)=R_(θ)/R₀.